Water-dispersible pressure sensitive adhesive compositions are used for many applications. For example they are used for fastening cloth on mammalian body coverings. Another field of use is papermaking and printing operations. There is required splicing of the end of one roll of paper to the beginning of another roll.
Examples of such water-dispersible pressure sensitive adhesives include U.S. Pat. Nos. 3,865,770; 4,413,080 and 4,569,960 (Blake), U.S. Pat. No. 3,441,430 (Peterson) and U.S. Pat. No. 2,838,421 (Sohl). Blake discloses water-dispersible pressure sensitive adhesives for splicing carbonless paper which comprise a blend of acrylate: acrylic acid copolymers and systems which include tackifiers, plasticizers and neutralizers. Peterson discloses aggressively tacky, water-soluble adhesives composed of a copolymer of a monocarboxylic acid and an alkoxy-alkyl ester, a plasticizer having at least one ether linkage, a cross linker and up to two parts of the hydroxide of a monovalent metal ion. The hydroxide is used to neutralize sulfuric acid generated during the decomposition of a potassium persulfate catalyst used for copolymerization. Sohl discloses a mixture of water-soluble polyvinyl carboxylic acid and a compatible hydroxy-polyalkylene permanent plasticizer which retains adequate tackiness and internal, cohesive strength even when exposed for a week or more at humidities in the range of 0 to 90% relative humidity.
A variety of other pressure-sensitive adhesives are also said to be water-soluble or water-dispersible. For example U.S. Pat. No. 4,413,082 (Gleichenhagen et al.) discloses a composition comprising a copolymer of acrylic acid butyl ester and vinyl carboxylic acid and a plasticizer. The copolymer which contains free carboxyl groups is neutralized almost completely by the addition of potassium hydroxide preferably in excess. Gleichenhagen et al. discloses that neutralization with potassium hydroxide normally brings about an excellent solubility of the pressure sensitive adhesive composition in water even at widely varying pH values. Solubility is measured in water without agitation. The only time reported to achieve solubility lies between 5 and 12 minutes. The adhesives disclosed by this reference are neutralized before being solubilized. However, in acidic media the salt resulting from neutralization will convert to the acid after a relatively brief time and the resulting polymer will precipitate out of the solution. Materials being repulped often remain in the hydropulper for several hours. As a result, materials which will precipitate with time are not repulpable as described herein.
U.S. Pat. No. 4,341,680 (Hauber et al.) discloses a mixture of a copolymer of a monomeric ethyl acrylate and a monomeric .alpha.,.beta.- unsaturated aliphatic monocarboxylic acid and a tertiary, ethoxylated n-alkyl alkane diamine, where from 50% to 90% of the acid groups of the monocarboxylic acid are neutralized.
European Patent Publication 0 352 442 discloses a pressure sensitive adhesive composition for papermaking tapes. The composition is a water-soluble contact adhesive based on a polyvinyl carbonic acid with at least one softener containing a hydroxyl group with a molecular weight below 1000 and curing agents used to process paper.
U.S. Pat. No. 3,152,940 (Abel et al.) discloses a pressure sensitive adhesive composition for photographic papermaking repulpable tapes. The preferred composition is an alkali-soluble copolymer of acrylic acid ester and acrylic acid and a water-soluble wax of hydroxy polyalkylene material which remains cohesive and tacky when exposed either to dry or humid atmospheric conditions.
U.S. Pat. No. 4,388,432 (Eskay) discloses a pressure sensitive adhesive composition for making water-dispersible tapes for papermaking. The adhesive employs a copolymer of an acrylic acid compound, which is a homopolymer of acrylic acid or a copolymer of acrylic acid and esters of acrylic acid or methacrylic acid, a polyhydric alcohol plasticizer-tackifier, and a minor amount of monobasic caustic. Amounts of the base higher than 2.5 percent cause the adhesive to become elastic in nature rather than tacky.
Another water-soluble pressure sensitive adhesive for papermaking tapes is disclosed in U.S. Pat. No. 3,096,202 (deGroot von Arx). Polyvinylpyrrolidone, a polyfunctional monomeric cross-linking agent, a compatible plasticizer, and an activator catalyst to induce vinyl-type polymerization are combined to produce a water-soluble, normally tacky pressure sensitive adhesive retaining its adhesiveness after subjection to elevated temperatures.
European Patent Publication 0 297 451 (Knutson et al.) discloses a hot melt adhesive activated by moisture which is water-soluble and alkali-dispersible to be recycled with paper products. The hot melt adhesive comprises an N-acryl-polyakyleneimine and an acid functional compound.
Another water-soluble adhesive said to be useful with paper products such as decals, labels, and bumper stickers is disclosed in U.S. Pat. No. 3,763,117 (McKenna et al.). The composition which has aggressive tack and aqueous alkali removability comprises a hydroxy bearing monomer, a softening acrylate or methacrylate comonomer, and optionally, a hardening comonomer of either vinyl esters of alkanoic acids, ethyl or methyl acrylates, ethyl or methyl diesters of maleic or fumaric acids, acrylonitrile, methacrylonitrile, styrene, or vinyl chloride.
A transfer or splicing tape is disclosed in U.S. Pat. No. 3,890,292 (Bohme et al.). The tape is formed with a water-dispersible adhesive composition of ionomeric copolymers of hydrophilic and hydrophobic monomers. The ionic monomer comprises a water-soluble alkali metal salt of an .alpha.,.beta.-unsaturated monocarboxylic acid. The water-soluble non-ionic monomers are polyalkylene oxide condensation products. The composition maintains excellent tack characteristics at both high and low humidities and has improved moisture stability properties.
Yet another water-dispersible pressure sensitive adhesive is disclosed in pending U.S. patent application Ser. No. 07/580,116 (Attorney docket number 45417USA8A) filed Sep. 10, 1990. The adhesive disclosed therein is formed from a blend of (a) 100 parts by weight of a copolymer of monomers of (1) about 50-85 weight percent of at least one monomeric acrylic acid ester of nontertiary alkyl alcohol having 2-8 carbon atoms, and (2) about 50-15 weight percent of a vinyl carboxylic acid, (b) about 10 to about 250 parts by weight of a water-dispersible or water-soluble plasticizer, (c) optionally from about 0 to about 100 parts by weight of an acrylate-compatible tackifier not irritating or toxic to mammalian skin; and (d) sufficient alkali metal hydroxide, if any, to neutralize from 0 to no more than 50 percent of acid moieties in the copolymer, to cause the adhesive to retain pressure sensitive adhesive properties during exposure to moisture and body fluids yet be dispersible in aqueous alkali solutions.
These pressure sensitive adhesives are soluble or dispersible under neutral or alkaline conditions. However, they typically have poor water-solubility under acidic conditions. Moreover it is not believed that they are repulpable under acidic conditions as described herein. Accordingly a need exists for a pressure sensitive adhesive which is repulpable (defined more fully hereinafter) under acidic conditions. For example, in papermaking and finishing operations, subsequent rolls are joined together with pressure sensitive adhesives (PSA) to form an endless web. The spliced areas are removed in the mill or at the end user and recycled in the manufacturing process. The paper pulp obtained from these materials (scrap) must not be contaminated with insoluble sticky substances, originating from the PSA.
Such adhesives have to meet several requirements in order to be useful:
In several processes the splice is subjected to elevated temperatures (200.degree. C. to 250.degree. C.) under high shear forces. The most severe conditions are typically observed in supercalender applications. Any slippage, even fractions of millimeters could lead to blocking and subsequent web breakage. PA1 A high cohesive strength is needed to withstand the high pressure/heat and resist sideways flowing (oozing) or penetration through the paper (bleeding). PA1 Adequate repulpability under a variety of pH conditions including both alkaline and acidic conditions (but especially acidic conditions). PA1 Good balanced adhesion properties (tack/peel/shear) PA1 Useable with light weight coated (LWC) papers.
The present invention describes a pressure sensitive adhesive composition which possesses these properties. The adhesive composition is especially suitable for preparing pressure sensitive adhesive tapes which can be used in the paper industry for splicing papers. However, the adhesive can also be used for such other applications as labels, masking tapes, protective coverings, stamps and the like.